Catalytic hydroprocessing refers to petroleum refining processes in which a carbonaceous feedstock is brought into contact with hydrogen and a catalyst, at a higher temperature and pressure, for the purpose of removing undesirable impurities and/or converting the feedstock to an improved product. Examples of hydroprocessing processes include hydrotreating, hydrodemetalization, hydrocracking and hydroisomerization processes.
A hydroprocessing catalyst typically consists of one or more metals deposited on a support or carrier consisting of an amorphous oxide and/or a crystalline microporous material (e.g. a zeolite). The selection of the support and metals depends upon the particular hydroprocessing process for which the catalyst is employed.
In general, during catalyst manufacture, it has long been known that use of a nitrate-containing salt as a source for depositing one or more metals on a support, in conjunction with an ammonium-containing component, will result in the presence of ammonium nitrate and nitric acid on the final product and waste stream. (See, Pemicone, Catalysis Today, vol. 34, pp. 535-547, 1997). This is highly undesirable as removal of the ammonium nitrate from the waste stream is difficult and costly. In addition, during calcination of the catalyst, ammonium nitrate will undergo an acid catalyzed rapid exothermic decomposition, which could be potentially hazardous to manufacturing equipment and will typically destroy the catalyst extrudates. (See, Wood and Wise, The Journal of Chemical Physics, vol. 23, pp. 693-696, 1955).
Ammonium nitrate and/or the nitric acid can be removed from the catalyst product prior to calcination by washing the product with water. However, this method produces an additional waste stream, and may not predictably remove a sufficient amount of the nitric acid to ensure the absence of an acid-catalyzed thermal event during calcination of the product catalyst.
In addition, where a modifying agent such as citric acid is employed during the deposition of the metals onto the support, a washing step would be undesirable, as this would result in the removal of a large concentration of the deposited metals and modifying agent as well.
Accordingly, there is a current need for a method for inhibiting rapid decomposition of ammonium nitrate on catalyst products in the calcination process, when a nitrate-containing composition is used in the deposition of active metals on the catalyst.